The present invention relates to methods for recovering hydrocarbons from a subterranean formation. The emphasis is on oil reservoirs. However, the methods are also applicable to gas, gas condensate, shale oil and tar sand formations. The fundamental technique used by the invention is to create a horizontal, high-permeability web at the bottom portion of a reservoir. This technique is made possible by recent developments in drilling and microwave technologies.
One of the simple ways to produce oil is to drill a single vertical well into an oil-bearing formation. Oil will be produced by the natural energy within the reservoir, such as expansion of gas cap or solution gas drive. After producing a fraction of the original oil in place (OOIP), it is becoming less economically attractive to deplete the reservoir by this primary recovery mechanism. A secondary production method is needed to push the oil through the formation. The basic scheme for secondary recovery is to drill a vertical well at certain distance away from the original well. Fluids such as gases or water are injected into one of the wells under relatively high pressure. Oil and other fluids are produced from the other well under relatively low pressure.
A drawback of using two vertical wells is poor volumetric sweep efficiency. Injected fluids tend to take a short path between adjacent injection and production wells that causes poor horizontal sweep. Gases tend to migrate through the upper portion of the reservoir. Water tends to migrate through the lower portion of the reservoir. These result in poor vertical sweep. Poor horizontal sweep results from well configuration. Gas gravity override and water gravity underride result from density differences between injected and reservoir fluids. Another phenomenon leading to poor volumetric sweep efficiency results from unfavorable mobility ratio. Viscosity of injected fluids is lower than that of the reservoir oil, which causes uneven frontal development known as viscous fingering. Even if there were no density and viscosity differences, injected fluids could still channel through more permeable strata, leaving a significant portion of the formation volume upswept. An oil recovery method mitigating these undesirable effects will enjoy increased oil recovery due to improved volumetric sweep efficiency.
Two of the methods used to improve volumetric sweep efficiency pertinent to this invention are gravity drainage and the use of horizontal wells. The following reference to prior arts will be focused on gas injection because gas injection is the preferred embodiment of this invention. After injected gas breaks through a production well, oil production rate falls off. Gas production rate increases. Excessive and early production of the injected gases is undesirable. It reduces the overall recovery, prolongs the operation, and imposes additional costs of processing and reinjecting produced gases, as disclosed in U.S. Pat. No. 4,368,781, to Anderson.
The most effective method to minimize gas production is gravity drainage. One of the earliest and probably the most widely used methods for gravity drainage is to perforate a production well at the bottom portion of a reservoir. Taking a horizontal cross section of the perforated zone, the wellbore acts as a single point pressure sink. There is a large pressure drop around the wellbore due to radial flow. The pressure drop is proportional to production rate. For a low-permeability reservoir, oil production rate is often limited by the parting pressure of the formation.
Numerous patent disclosures are related to horizontal wells. See Allen U.S. Pat. No. 4,410,215, Brannan et. al. U.S. Pat. No. 5,273,111, Brown et. al. U.S. Pat. No. 4,718,485, Huang et. al. U.S. Pat. Nos. 4,702,314, 5,065,821 and 5,320,170, Mullins et. al. U.S. Pat. No. 4,385,662, and Shu et. al. U.S. Pat. No. 4,598,770. A horizontal production well acts as a linear pressure sink. It provides a relatively large area for flow that results in smaller pressure drop and improved volumetric sweep efficiency.
The above patents include methods of spatial arrangements of horizontal and vertical wells. The purpose of this invention is not to disclose new well arrangement patterns. Instead, it is to adapt high-permeability web to any well arrangement configuration as known in the art. Therefore, only an inverted 5-spot pattern and a pattern of parallel horizontal wells are used as preferred embodiments. Inverted 5-spot patterns are found in many oil fields. Such a pattern consists of one vertical injection well and four vertical production wells. A modified inverted 5-spot pattern is disclosed in U.S. Pat. No. 5,320,170 to Huang et. al., adding four horizontal wells along the sides. The other preferred embodiment teaches drilling laterally and vertically staggered horizontal wells as disclosed in U.S. Pat. No. 5,237,111 to Brannan et. al.
Two of the technologies are modified and used to create a high-permeability web around a vertical or horizontal well. One is high-pressure water jet as disclosed in U.S. Pat. No. 5,413,184 to Landers. The other is high-power microwave energy as disclosed in U.S. Pat. No. 5,299,887 to Ensley. The high-pressure water jet cuts a channel into a formation at a distance of 200 feet and beyond. The contemplated application of the technique is to cut additional branches of channels at different locations and directions. For the microwave technology, an antenna is lowered into a production well to the bottom portion of an oil-bearing formation. The antenna generates electromagnetic waves at selected frequencies. The frequencies used for this application are in the Ghz range, or microwaves. High-power microwave beams aimed horizontally will penetrate the formation up to 100 feet. The microwave frequencies are selected to maximize vaporization of hydrocarbons and water in the porous media. Because the high-power microwave energy is delivered rapidly, vaporization is completed in seconds or minutes. The sudden generation of large amounts of gases will fracture the formation, resulting in a permeability increase of several orders of magnitude, along the path of microwave penetration. Produced gases return to the production well through the high-permeability channel. The direction of microwave penetration is rotated until a desired fracture pattern has been developed. The above technologies produce a horizontal, high-permeability web around the wellbore. Because the entire high-permeability web acts as the pressure sink, the pressure drop is small.
Gravity drainage is also used in conjunction with any enhanced oil recovery (EOR) method. The purpose of EOR is to improve mobility control and displacement efficiency. Mobility control mitigates viscous fingering. Improved displacement efficiency reduces residual oil saturation in the pores that have been swept by the injected fluid. Common EOR methods include thermal (e.g., steam and combustion), miscible (e.g., CO.sub.2), and chemical (e.g. surfactant and polymer). Examples of recent disclosures of EOR methods used with gravity drainage are thermally assisted gravity segregation disclosed in U.S. Pat. No. 5,503,226 to Wadleigh; and horizontal well gravity drainage combustion process disclosed in U.S. Pat. No. 5,456,315 to Kisman, et. al.
Additional factors affecting gravity drainage are properties of the reservoir and injected fluids, stratification and flow characteristics of the porous media, oil field facilities, operation strategies, and process economics. These factors will also affect the implementation of high-permeability webs.